Parison Thickness Control

Parison control means dynamically controlling thickness of Parison to get a uniform thickness container on an Extrusion Blow Moulding machine. It helps to maintain the required thickness axially and radially on the product made out of Extrusion Blow Moulding (EBM).
What is Parison?
Now what is a Parison? It is the molten tubular form made on an Extrusion Blow Molding machine by extruding thermo plastic resins. This viscous melt resin called Parison is put into two halves of Blow Molds and shaped to the form of cavity of mould by blowing compressed air.
Why control required?
Feedback SensorWhen the Parison comes out of the die head of the EBM machine it will be in the molten state. Due to the self-weight the Parison will elongate resulting in a thinner top portion compared to the bottom portion. This will cause uneven thickness in the article made. Also the complex geometries of the product to be molded necessitate varying the parison thickness to get more uniform product wall thickness.

The advantages:
· Uniform article wall thickness and hence improved quality due to lesser thermal stresses during processing.
· Even distribution of thickness and hence possibility for reduction in weight.
· Reduction in top and bottom wastage and hence higher production.
· Reduced cooling time due to uniformity of thickness and hence higher production.
Different types of Parison Control
Relay logic control (timer based)
In single stage parison control the thickness is adjusted once in a parison. This can be either thick-thin or thin-thick type. In three stage control the thickness is adjusted three times in a parison (thin-thick-thicker or thick-thin-thicker type). This control is achieved by a simple On-Off hydraulic valve operated by a set of timers and relays.
Closed loop control (Multi-point Parison Programmer)
Closed loop electro-hydraulic control system can be fitted on the machine to control the parison thickness at several points (30 times or 100 times or more). Since this is a closed loop system the accuracy is much better than ordinary system and the resulting container will have most uniform thickness.
Axial Wall thickness Control
All the above described systems are for thickness control in axial direction of Parison. This is the most common Parison controls.
Radial Wall thickness Control (PWDS)
Other than the axial wall thickness control, for some specific and complex products, thickness control in radial direction is required. This is achieved by transforming the uniform annular space through which parison comes out to elliptical shape.
MOOG
MOOG is the pioneer in Parison programming or Parison control.
Other Providers
Other than moog several other parison control systems are available. They include:
· B & R, Austria
· Gefran, Italy
· Beckoff, Germany
· Yuken, Japan
· Barber & Colman, USA
· Seitlure Systems, India
Time based and position based Control
In the continuous extrusion blow moulding time based systems are used. That is the parison thickness is controlled on a definite interval of time. Here the total time for one parison cycle is divided by the number of points to calculate the time interval.
In accumulator type extrusion blow moulding either a time based or a position-based system can be used. In position based system a potentiometer senses the position of the push out (accumulator) cylinder stroke and programs the profile points with respect to the position. In time based system the profile points are programmed with respect to the time taken for parison push out. To overcome the variations in time the controller automatically updates the time in each cycle and uses it for programming the next cycle.
---------------------------------